1. Field of the Invention
The present invention relates to measurement-while-drilling and logging-while-drilling technologies used in the oil and gas industry, and more specifically to downhole mud pulse telemetry assemblies.
2. Description of the Related Art
In downhole drilling operations, tools are lowered deep into the ground to perform various functions, such as drilling, measuring related data, and logging the data. Measurement-while-drilling allows for the surface acquisition of downhole data during drilling. Boreholes may be thousands of feet in length, and may encompass different geological rock formations. Due to the length of the drill string, communication between surface equipment and downhole tools is limited during drilling applications. In modern measurement-while-drilling systems, information is telemetered to the surface primarily through either mud pulse, electromagnetic, or acoustic telemetry systems. These telemetry systems may transmit data regarding the location, orientation, and inclination of the drill string, the type of rock formation that is being drilled, the rotational speed of the drill string, etc.
Mud pulse telemetry systems are commonly used to communicate between a downhole tool and the uphole operator during drilling applications. Data to be transmitted is encoded into a predetermined encoding scheme. In mud pulse telemetry systems, a pulser is used to generate surges, or pulses, in the drilling fluid, generically known as mud, which is flowing through the drill string. The pulser generates pressure pulses by constricting a flow path in the fluid in the borehole. The constrictions are created and then released in the drill string with a specific timing to generate patterns to encode data based on the timing of the pressure pulses.
A common type of pulser is a hydraulic, or tensor, pulser. In a hydraulic pulser, pulser driver circuitry activates a solenoid. The solenoid actuation causes a poppet valve to open, which causes a signal shaft to move and create another restriction at the bottom of the tool. This restriction causes a pulse to be generated through the fluid.
The pressure pulses propagate through the fluid from the downhole tool to the surface. Pressure sensitive transducers at the surface can detect the encoded pulses and decode the pulses to arrive at the original data. The real time capture and transmission of data allows operators at the surface to make real time decisions regarding drilling operations. The mud pulse telemetry communication may be bidirectional, and surface equipment may generate pulses that are telemetered downhole to steer or control a downhole drill string.
Mud pulse telemetry systems must have fluid flowing through the system in order to generate pressure pulses and telemeter data. In order to sense whether drilling fluid is being circulated, vibration sensitive devices are incorporated into the pulser driving circuitry. These vibration sensitive devices, generically known as flow switches, send a signal to the measurement-while-drilling control circuitry, generically known as the microprocessor unit (MPU), when the flow state changes from off to on.
For the effective operation of downhole tools, accurate and robust flow switches are essential. Mud pulse telemetry tools operate on battery power, so unnecessary triggering of the pulser should be avoided to conserve power. Further, without flow switches to accurately determine the flow state, the pulser could trigger without fluid flow and fail to convey a pulse to the surface, resulting in the potential loss of encoded data. The following invention relates to the development of a “smart” flow switch which implements algorithms to accurately and robustly determine the flow state.